Embodiments of the invention relate generally to magnetic resonance (MR) imaging and, more particularly, to MR imaging using a waveguide to transmit radio frequency (RF) pulses of an MR scan sequence.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue partially align with this polarizing field, to the extent permitted by thermal agitation. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. The transverse moment precesses at the Larmor frequency; and the signal emitted by the excited (precessing) spins after the excitation signal B1 may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In typical MR systems, as is known in the art, RF coils are used to generate RF signals that generate the excitation signal B1 to tip and excite the spins in the tissue being imaged. These signals are strictly localized in the vicinity of the antenna (or coil) which produces them. In another type of recently introduced MR system that reflects a new paradigm for MR transduction, a hollow, cylindrical tube within the bore of the main magnet may under certain conditions, be used as a waveguide to propagate the excitation RF signals, in the form of travelling waves and indeed to receive the RF emissions from the resulting transverse magnetization. A major benefit of travelling wave MR is the ability to produce RF excitations along the entire length of the human body, using only a single transmitter mounted at an extremity (head or foot) of the subject to be imaged.
In a typical MR system, the geometries of such a cylindrical tube result in a cutoff frequency for the waveguide at or near 300 MHz for its principal mode of operation i.e. the TE11 mode, as is known in the art. For an MR system having a main magnetic field strength of 7.0 T (corresponding to a Larmor frequency of 300 MHz for protons), travelling wave MRI near the cutoff 300 MHz is therefore practicable. Travelling wave MR imaging has heretofore been applied such very high Larmor frequencies (e.g., 300 MHz). This value of the cutoff frequency is fortuitously determined by the diameter of the conductive gradient shield, which, as is known in the art, forms a conductive cylinder between the radiofrequency body/volume transmit coil, and the tube on which are mounted the imaging gradients. That is, the cutoff near 300 MHz is a consequence of the shield diameter, which is chosen for reasons not related to selection of a particular frequency.
However, most MR systems designed for use with live imaging subjects have magnetic field strengths less than 7.0 T. For example, many MR systems are designed with magnetic field strengths of 3.0 T or 1.5 T. To use a waveguide in a 3.0 T or 1.5 T MR system in its principal (e.g., transverse electric) mode of operation, the waveguide would have to have a diameter larger than the main magnet bore of a typical MR system.
It would therefore be desirable to have a system for MR imaging using a waveguide in a typical MR system having a magnetic field strength less than 7.0 T.